Flu
Why flu season happens and how to help protect yourself
Learn why flu season peaks in colder months, who’s most at risk, and why getting a yearly flu shot is important to help protect yourself
Dr. David Weber · August 13, 2025
6 min read
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Key takeaways
- Flu season typically runs from fall to early spring, peaking in winter.
- Cold weather, indoor crowding, school sessions and holidays contribute to the spread of the flu.
- Anyone can get the flu, but certain groups are at higher risk.
- The flu vaccine is updated yearly to match circulating strains.
- Getting vaccinated each year can help reduce your risk.
What is flu season, and why does it matter?
Every year, as the weather cools and people spend more time indoors, flu season begins. But what exactly is flu season, and why does it come back every year?
Influenza, or the flu, is a contagious respiratory illness caused by influenza viruses. It can lead to mild to severe illness and, in some cases, serious complications including death. People can have fever, cough, sore throat, body aches, and fatigue. People can be contagious (i.e., transmit the flu virus to others) even before they know they are sick. While most people recover on their own within a week, the flu can be dangerous—especially for children under five, older adults, and people with certain health conditions.
Understanding flu season can help you take timely steps to help protect yourself and those around you.
When is flu season?
In the Northern Hemisphere, including the United States, flu season typically begins in October, peaks between December and February, and lasts through April. However, the exact timing can vary by region and year.
In the Southern Hemisphere, flu season usually occurs from April to September, aligning with their winter months. In tropical regions, flu activity can occur year-round but may still have seasonal peaks.
Why do we have a flu season?
Several factors contribute to the seasonal nature of the flu:
- Colder Weather: Flu viruses survive and spread more easily in cold, dry air.
- Indoor Crowding: People spend more time indoors during colder months, increasing close contact and the chance of transmission.
- School Sessions: Schools are crowded with both children and adults, who are at risk for the flu, and flu season typically coincides with the school year.
- Holidays: During holidays such as Thanksgiving, Christmas and New Years people often travel, have family get-togethers, and attend religious events, which can increase the risk of getting the flu from others.
- Weakened Immune Systems: Less sunlight in the winter can contribute to lower levels of vitamin D, which may affect immune function.
Which of the following groups is at higher risk of flu complications?
Adults over 65, children under 5, and people with underlying medical conditions
Correct
Adults over 65, children under 5, and people with underlying medical conditions
Not quite
Correct answer: Adults over 65 and children under 5, and people with underlying medical conditions
Are some flu strains worse than others?
Influenza viruses are constantly changing. Each year, different strains circulate—some more severe than others. The flu vaccine is updated annually to target the strains expected to be most common.
Some years, the dominant strain may cause more severe illness or spread more easily, which can lead to a more intense flu season.
Who gets the flu?
Anyone can catch the flu, but some groups are more vulnerable to complications:
- Adults 65 and older
- Young children, especially under the age of 5
- Pregnant women
- People with certain chronic conditions, such as asthma, diabetes, or heart disease
- People with weakened immune systems
Influenza viruses are constantly changing. Each year, different strains circulate—some more severe than others.
How to protect yourself from the flu
The best way to help protect yourself and others is to take preventive steps:
- Get vaccinated every year if older than 6 months of age.
- Wash your hands frequently with soap and water or use a waterless alcohol-based hand hygiene product.
- Avoid close contact with people who are sick.
- Stay home if you’re feeling unwell.
- Cover your coughs and sneezes with a tissue or your elbow.
If you are at high risk for serious disease (e.g., you are being treated for cancer and immunocompromised), consider wearing a mask outside your home or when inside and in close proximity to other people.
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The flu vaccine: what you need to know
The flu vaccine can help protect against seasonal influenza. The flu vaccine helps lower the chances of getting sick and reduces the risk of severe complications that could lead to hospitalization. Designed to help your immune system recognize and fight off the most likely strains of the virus each year, the vaccine is available as a shot or nasal spray. Flu vaccination is recommended for everyone age six months and older.
- Timing matters: the Centers for Disease Control and Prevention suggests getting vaccinated by the end of October, before flu activity typically peaks. However, it’s still beneficial to get a flu shot later in the season if you miss that window.
- Co-administration: The flu vaccine can be given at the same time as other respiratory virus vaccines, including those for COVID-19 and RSV. This can be a convenient way to help protect yourself against more than one disease with a single visit. Read more about getting more than one vaccine 'Getting More Than One Vaccine: What You Need To Know' article.
- Reformulated Annually: Because flu viruses constantly evolve, the vaccine is reformulated annually. Each year, researchers analyze data from around the world to predict which strains are most likely to circulate. Based on this data, the vaccine is updated to include protection against the most prevalent strains. Most flu vaccines help protect against three types of influenza viruses: two influenza A viruses (H1N1 and H3N2) and one influenza B virus. This is why getting vaccinated every year can be beneficial. Importantly, more than 1 type of flu virus spreads each year, thus unvaccinated people can get the flu more than once in a single respiratory viral season.
Special considerations
Some people may need special considerations:
- Older adults and adults with a solid organ transplant (e.g., kidney) (especially those 65 years and older) may benefit from high-dose or adjuvanted versions, which include an added ingredient to help strengthen the body’s immune response. The Centers for Disease Control and Prevention specifically recommends one of these “enhanced” vaccines for older people and adults who have had a transplant.
- Pregnant individuals are encouraged to get vaccinated to help protect themselves and their unborn child. Pregnant women should discuss with their healthcare provider about receiving the flu vaccine earlier in the Fall.
- People with a weakened immune system should receive inactivated flu vaccines. Live attenuated vaccines (like the nasal spray) are generally not recommended.
- People with egg allergies can opt for egg-free formulations but the Centers for Disease Control and Prevention states that such people may receive any age-appropriate flu vaccine.
It’s also important to note that many people use the term “flu” to describe any respiratory illness during the winter months. However, the flu vaccine only protects against influenza viruses. While there are vaccines for other respiratory viruses—like COVID-19 and RSV—not all respiratory illnesses are preventable by vaccination.
Stay ahead of the flu
Flu season may come around every year, but that doesn’t mean you have to face it unprepared. By understanding how and when the flu spreads—and taking simple steps like getting vaccinated every year—you can help protect yourself. Visit VaxAssist.com to check your eligibility for respiratory vaccines, schedule your flu vaccine appointment and learn more.
Frequently asked questions
This article was medically reviewed by Dr. David Weber, a compensated expert engaged by Pfizer to ensure the information provided is medically accurate and current. The content is for educational purposes only and is not a substitute for professional medical advice.
References
5 myths about the flu vaccine. The World Health Organization. Accessed July 7, 2025. https://www.who.int/news-room/spotlight/influenza-are-we-ready/5-myths-about-the-flu-vaccine.
About Influenza. The Centers for Disease Control and Prevention. August 13, 2024. Accessed July 7, 2025. https://www.cdc.gov/flu/about/index.html.
Ao T, Kikuta J, Ishii M. The effects of vitamin d on immune system and inflammatory diseases. Biomolecules. 2021;11(11):1624. https://pmc.ncbi.nlm.nih.gov/articles/PMC8615708.
Davis RE, Rossier CE, Enfield KB. The impact of weather on influenza and pneumonia mortality in New York City, 1975–2002: a retrospective study. Viboud C, ed. PLoS ONE. 2012;7(3):e34091.
Influenza (seasonal). February 28, 2025. The World Health Organization. Accessed July 7, 2025. https://www.who.int/news-room/fact-sheets/detail/influenza-(seasonal).
Key Facts About Seasonal Flu Vaccine. The Centers for Disease Control and Prevention. September 17, 2024. Accessed July 7, 2025. https://www.cdc.gov/flu/vaccines/keyfacts.html.
Selecting Viruses for the Seasonal Influenza Vaccine. The Centers for Disease Control and Prevention. March 12, 2024. Accessed July 7, 2025. https://www.cdc.gov/flu/vaccine-process/vaccine-selection.html.
Treating Flu with Antiviral Drugs. The Centers for Disease Control and Prevention. September 11, 2024. Accessed July 23, 2025. https://www.cdc.gov/flu/treatment/antiviral-drugs.html.